Polymers containing heteroatoms along the backbone play an ever-increasingly important role in modern society, and the variety of such polymers continues to expand at a high rate. For example, poly(ethylene terephthalate) (i.e., poly(oxy-1,2-ethanediyl-oxycarbonyl-1,4-diphenylenecarbonyl), or “PET”) is a widely used engineering thermoplastic for carpeting, clothing, tire cords, soda bottles and other containers, film, automotive applications, electronics, displays, etc. The worldwide production of PET has been growing at an annual rate of 10% per year, and with the increase in use in electronic and automotive applications, this rate is expected to increase significantly to 15% per year.
Polymers with heteroatoms along the backbone are commonly prepared using an addition-type polymerization mechanism, in which monomers react to form dimers, which can in turn react with other dimers to form tetramers. This growth process is allowed to continue until polymers with the desired molecular weight are formed. Unfortunately (and unlike the alternative chain-growth polymerization mechanism), obtaining high molecular weight polymer using this mechanism requires carrying the polymerization reaction to very high conversion.
A frequently-used method for commercial synthesis of (PET) involves a two-step transesterification process from dimethyl teraphthalate (DMT) and excess ethylene glycol (EO) in the presence of a metal alkanoates or acetates of calcium, zinc, manganese, titanium, etc. This first step generates bis(hydroxy ethylene) teraphthalate (BHET) with the elimination of methanol and the excess EO. The BHET is heated, generally in the presence of a transesterification catalyst, to generate high polymer. This process is generally accomplished in a vented extruder to remove the polycondensate (EO) and generate the desired thermoformed object from a low viscosity precursor.
Some polycondensation reactions, such as the commercial method of synthesis of PET described above, require polymerization catalysts. Such catalysts may be difficult to prepare, may be unstable to long-term storage, or may require stringent reaction conditions to provide polymer. Moreover, these catalysts are immortal, limiting the versatility of the widely used mechanical recycling, because at high temperatures the residual catalyst causes molecular weight degradation. This limits the use of these recycled products to secondary applications (i.e., carpet, playground equipment etc.).